Pros & Cons of a Rotary Engine

by Dennis Hartman

Although not common in modern automobiles, rotary engines offer a drastically different alternative to conventional reciprocating piston combustion engines. While automakers who use a rotary engine are quick to point to its many advantages, there are also certain drawbacks. The pros and cons of a rotary engine explain why it is preferable in certain applications, even though it is not a standard offering in most automobiles.

Mechanical Operation

A rotary engine uses a triangular-shaped rotor to divide the space inside the engine, enabling a standard four-stroke cycle of intake, compression, ignition and exhaust. The moving rotor transports fuel to the various engine compartments for each leg of the cycle. In this way, it resembles a reciprocating piston engine. Rotary engines can be built with any number of rotors, much like the multiple number of cylinders offered in piston engines. The rotors engage a drive shaft, which then powers the vehicle's drive mechanism (the propeller of a plane, or wheels of a car).

Simplicity

One of the major advantages of a rotary engine is its mechanical simplicity. A rotary engine contains far fewer parts than a comparable piston engine. This may decrease the cost of design and manufacture. This also leads to decreased weight. Compared to standard reciprocating piston engines, rotary engines contain no valves, camshaft, rocker arms, timing belts or flywheel. All this means decreased weight, fewer opportunities for malfunction and easier repair. When rotary engines were first developed, they were used to power aircraft, taking advantage of the rotary engine's high power-to-weight ratio.

Other Advantages

Due to its revolutionary motion, a rotary engine operates with less vibration than a piston engine. This allows rotary engines to be tuned to run at higher rpm, thus producing more power. Another advantage of the rotary engine is that in the event of a breakdown, the engine will not seize up. Loss of compression or other common sources of engine failure will likely result in a severe loss of power, but the rotary engine will continue to run for a time, whereas a piston engine will halt operation immediately under similar stresses.

Disadvantages

Rotary engines do contain design elements that lead to operational disadvantages as well. Leakage between engine chambers is common and generally results in a loss of efficiency over time. Also, rotary engines are not expected to last as long as traditional reciprocating piston engines. Other design elements lead rotary engines to purposefully burn engine oil in small amounts, thus requiring that oil levels be frequently checked and added to. The increased maintenance and short lifespan make rotary engines preferable for specific applications other than mass-market cars.

Applications

Since rotary engines are unlikely to seize during operational failure, they are a much safer choice for aircraft, allowing the pilot of a plane with a failed engine time to land safely. Rotary engines are also used in sports and racing cars, most notably in Mazda's RX series of sports cars. This is mainly due to the rotary engine's ability to run at high rpm and produce more power over a shorter duration. Other applications that take advantage of the rotary engine's smooth operation include go-karts, personal watercraft, gas generators, motorcycles and chainsaws.